Project objectives are based on light and electron microscopic analyses of the chromosome complements of Lemur macaco, Lemur fulvus and intra- and interspecific lemur hybrids bred and housed at the Duke University Primate Center. The hybrids, both natural and labaoratory bred, provide models for analyses of Robertsonian chromosomal rearrangements: 13 different metacentric/acrocentric combinations are available for study. Mitotic karyotypes are established by chromosome banding. Synaptonemal complex (SC) karyotyping of surface-spread pachytene spermatocytes is used to study meiotic synapsis and desynapsis, and disjunction and distribution are followed in acetic-alcohol preparations. SC analyses of trivalent synapsis in hybrids permit the mapping of individual chromosome breakpoints and suggest that the mode of formation of Robertsonian metacentrics, synaptic timing, and the ultimate geometry of trivalent configurations, or a combination of these factors, may influence the rate of malsegregation differentially in heterozygotes. These propositions are tested by correlating results from SC studies with counts of aneuploid metaphase-anaphase II cells, and with analyses of the number and normality of the sperm of hybrids vs. those of homomorphic individuals. Breeding experiments provide ultimate tests of hybrid fertility and reproductive efficiency, and facilitate the isolation of individual heteromorphic trivalents.